A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
EUV radiation may be produced using a radiation source arranged to generate an EUV producing plasma. An EUV producing plasma may be generated, for example, by exciting a fuel, for example liquid tin, within the radiation source. The fuel may be excited by directing a beam of initiating radiation, such as a laser beam, at a target comprising the fuel, the initiating radiation beam causing the fuel target to become an EUV generating plasma.
It is desirable to provide an EUV radiation source which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.